1. Field of the Invention
This invention relates to a method of manufacturing felts for vinyl flooring or gasketing material, and specifically to the use of a latex binder for papermaking fibers which also removes contaminating dissolved ions from the process water.
2. Brief Description of Prior Art
Chrysotile asbestos has found widespread use in the manufacture of felts for vinyl flooring and gasketing materials. The fibers of chrysotile have a cationic electrostatic charge. It is convenient to use a styrene-butadiene rubber latex, the particles of which are negatively charged, to bond the fibers into a uniform sheet. Due to the difference in electrostatic charge, the particles of latex are quantitatively adsorbed on the fibers within seconds.
Unfortunately, chrysotile contains mineral salts which dissolve in the process water during the mechanical dispersion of the fibers. As these minerals become more concentrated in the process water, the drainage of water from the asbestos stock is reduced, slowing the felt-making machinery. Fresh water must then be added to the process water. As a result, part of this process water must continually be discarded. Closing of the process water system would result in a savings of fuel and raw materials and eliminate objectionable effluents.
Fibrous substitutes for asbestos which have been proposed, such as cellulose, fiberglass, rockwool and polypropylene are relatively inert and do not readily adsorb latex particles. These asbestos substitutes must be pre-treated with agents such as alum or polymeric flocculants and/or used with mineral fillers such as kaolinite or wollastonite to promote the precipitation of latex particles in the formation of a felt. The accumulation of dissolved mineral salts in the process water will be as much a hindrance to the closure of the process water system with these asbestos-free compositions as it is with asbestos-containing felt compositions.
Conventional styrene butadiene latices used in felt-making are usually prepared in a batch polymerization. All of the monomer is in the reactor at the start of the polymerization and must be emulsified by copious amounts of surfactants. These surfactants are also needed to prevent coagulation of the latex particles during storage and handling. Adsorption of the latex particles onto asbestos in the felt-making process releases the surfactants to the process water adding to the dissolved salts originating from the mineral component. In addition, these surfactants, particularly those of non-ionic type, enter the air-water interface causing the formation of a stable foam, requiring the continuous introduction of an antifoam composition.
If it were not for the surfactants, the surface of the particles of the styrene-butadiene latices, containing copolymerized carboxylic and sulfate acid groups, would provide ion exchange capacity which would aid in the purification of process water. "Low Emulsifier" synthetic styrene-butadiene latices were prepared in a two-step polymerization in U.S. Pat. No. 3,784,498. Stability was obtained by pH adjusting the latices in the second step with ammonia to between 7 and 10. Styrene-butadiene latices prepared by a similar two-step polymerization with N-methylol acrylamide in U.S. Pat. No. 3,882,070 had a final pH of greater than 5. The pH of these latices is too high to be useful for precipitation of cationic fibers in felt manufacture and to adequately demonstrate the ion exchange capability.